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Thursday, December 30, 2010

These next pictures are of the Pelecypod Goniophora. The first picture shows a smaller specimen with the tell tale shell markings of regularly spaced growth lines that form a right angle across a longitudinal axis line.

These next two pictures show a larger individual that is just an internal shell cast. You can still see the signature shell shape reflected in the cast.

Casts of the internal part of the shells are neat to look at because you can see how the hinge and shell edges were shapes and fit together. This picture almost looks like one of those Rorschach images.

Tuesday, December 28, 2010

More Mahantango finds from the flats that have accumulated in my attic from the 2010 collecting year.

This is a cast of the internal space within a Nucula sp. shell. It was two complete valves that were buried articulated as in life. Mud seeped into the empty space between the shells, where the animal once lived, and hardened. After the exterior shell dissolved away the cast was left behind.

I like that the hinge teeth are preserved as part of the cast.

Nuculites is a common fossil within the Mahantango and below are some more examples. The most diagnostic feature for identifying this genera is the impression of the clavical groove that is found near the edge of the shell.

This was an oddball piece that I kept because it kind of looked funny. It is a large cephalon from a Dipleura trilobite but where the eye stalks should be are two chonetid type brachiopods. It's likely that this came about after the cephalon was flipped upside down by currents and settled this way on the sea floor. Later currents swept the brachiopod shells into the hollow trilobite skeleton fragment.

Sunday, December 26, 2010

I'm going through some boxes of finds from the past collecting year and photographing what I've found. Part of this is documentation, part is to give me ideas for this blog.

I was very happy with this find, it's a complete Orthonota sp. pelecypod from the Mahantango. Most times I find this fossil with one or both valves as impressions in the rock. To find a loose specimen is really cool. I profiled Ortonota in a previous post as well.

My love of Pleurodictyum is well documented, but here are a two different specimens that caught my eye while parsing through the boxes. You can see casts of the internal cavities of Pleurodictyum but the most interesting thing is the fact that they grew on a large Pelecypod shell. This is a common find within the Mahantango and shows that the seabed was not hard enough for the coral to establish a colony.

This is perhaps the best Mucrospirifer that I've yet found in the Mahantango. It's just a mold of the shell but you can see the extents of the elongated hinge line. Considering that it is very difficult to get a whole specimen of Mucrospirifer with it's "wings" intact, this is not a bad specimen.

Friday, December 24, 2010

A common Pelecypod found in the Mahantango formation is Leiopteria. As is often the case, the example below is of an internal cast and external mold found recently.

This is how the piece was found lying on the surface. You can see the internal cast resting within the outline of the external shell mold.

Here I have removed the internal cast. All the loose, brown material is from small bits of the sediment that were trapped within the shell. Once the shell material dissolved away they were left behind. This is a common occurrence and tends to lend a brown color to your fingers once handled. Note the extended hinge line visible on the upper right. This anatomical feature helps place the animal in the same family as modern day Pearl Oysters.

These last two pictures are of the internal cast. You can see some internal shell details preserved such as the pimple like muscle attachment points seen near the tip.

Wednesday, December 22, 2010

The Arkona formation is the basal unit present at the collecting locale known as Hungry Hollow along the AuSable river neqr Strathroy, Ontario, Canada. The formation is primarily composed of a shale that has a high percentage of clay. The formation breaks down quickly in the presence of water and when it rains the slopes that form on the formation become very slick and sticky. There are some terrific fossils that can be found it it but you have to quarry the shale when it's fresh and NEVER GET IT WET!

Want see what happens to the Arkona formation shale once it gets wet? I learned a good lesson not long after I got home from a recent trip to Ontatio. I'd brought back a piece of the shale with a fossil in it as an example of the formation. When I found the piece it had a little bit of dirt on it and I thought I could just rinse it off. Before the rinse you can see a partially pyritized Bactrites and a pyritized worm trail in a fairly solid piece of shale.

And here it is after I quickly splashed it in a bucket of water and then took it out. The transformation took place before my eyes like a science experiment and I am not kidding when I tell you that this picture was taken maybe a minute after the piece got wet.

Side view of the once solid shale

Lucky for me this was not a prize specimen or something I really wanted. Within a few minutes the whole rock was cracking apart as the clay absorbed the water and expanded. Since the clay quickly absorbs the water, only the surface of the formation is like this. If one were to dig into the slopes that line the river and extract chunks of the shale it would be stable. I'm told by my friend Joe (click here to see some of the Crinoids he's found from Hungry Hollow) that the only way to explore or prep the rock is with tools and then it needs to be sealed.

Epilogue: Here is a picture taken a few weeks after I left the rock outside to finish eroding. You can see it's lost most of it's color and shape as well as mass as the rain has washed it away. The pyritized Baculite has since fallen off to the side.

Monday, December 20, 2010

One of my favorite finds from my recent trip to NY was the discovery of whole Tropidoleptus carinatus shells. In a prior post I showed some examples of the species from the Mahantango. Up recently I had never found a complete shell with both valves intact and on display. My stop to collect from the Kashong shale member of the Moscow formation proved to be very fruitful and has changed my view of Tropidoleptus. Tropidoleptus is a good index fossil for the Devonian as it is first found in Lower Devonian rocks and disappears at the Fransian/Famennian boundary of the Upper Devonian. Below is a rotation of a typical Tropidoleptus shell found in the Kashong shale.

Pedicle valve

Front

Brachial valve

Rear

Profile

You can see that it has an overall flat profile with a slightly curved profile. Notice also that the Brachial valve is flat to convex, following the contour of the Pedicle valve. This is one of the first features that had me confused when I first found these fossils.

When one looks up the classification of Tropidoleptus it is currently placed in the Terebratulida order. This has not always been the case and has at times been a part of the Orthida and Strophomenida orders (depending on who was doing the classification). According to the Paleobiology Database, Tropidoleptus currently is ranked as the type (and only) genus of the family Tropidoleptidae. This is owing to the unusual nature of the fossil. It shares features with several different groups and defies being pigeonholed. To me, the overall appearance of the shell looks like a Strophomenid, such as Strophodonta or Rafinesquina, with their flattened, curved shells. But the wide, thick rays that line the surface of the shell imply a Rychonellid origin.

Another aspect of the fossil that has troubled me was that there seems to be some variability to the look and shape of the shell. Below is a rotation of a smaller (and presumably younger) shell.

Pedicle valve

Front

Brachial valve

Rear

Profile

You can see that the smaller shell has a long hinge line which is close to the widest part of the shell. As the animal grew larger this trend becomes more exaggerated and the sides of the shell expand well beyond the width of the hinge line (as can be seen in the shell illustrated at the start of this post). Also note how the Brachial valve has strong concentric growth lines that ornament the shell like a sun rising over the horizon. If you were to find the Brachial and Pedicle valves separated then one could confuse them for different species. It's been a bit difficult for me over the years to accept that these disparate valves and shell width/age differences all added up to the same species. Regardless of what it looks like I'm happy to have found specimens that allow me to fully visualize the extinct animal.

Just for illustrative purposes: here is a shell from the Kashong Shale with some of the Pedicle valve missing which allows the muscle scars on the interior of the Brachial valve to be seen.

And here are a couple of examples of Tropidoleptus as found in my local Mahantango formation:

Saturday, December 18, 2010

Another find, from the new locality I mentioned in the last post, were these pieces of Bryozoans.

They could be corals but the diameter of the living chambers seem to be too small.

I was pleasantly surprised to see that the mineralized (calcite) skeleton was still present and had not been dissolved away.

Although some specimens had been affected by ground water and only showed a small amount of the skeletal structure.

The colonies that I found seem to form short, fat finger like shapes and I think they fall into the Trepostomida order. They were only found within a certain area of the pit and likely were part of a layer or lens of stable seabed where animals could colonize. I have not been able to find any references to Bryozonas that look like these specimens yet. I've found specimens of similar looking Bryozoans from the upper Silurian called Rhombotrypa and Monotrypa but they don't seem to extend into the Devonian. Likely I will not find the truth out unless I were to thin section the specimens and examine them under a microscope.

Thursday, December 16, 2010

Recently I took a trip up into Schuylkill County to check out a new (to me) pit that exposes the Mahantango formation. The fossils were somewhat sparse and seemed to be concentrated in thin layers or lenses. I was pleasantly surprised to find some interesting Crinoid specimens.

These are a couple of Crinoid arms. You can see the segmentation of the arms and some of the pinnules. This piece also shows the typical preservation for the Mahantago formation with the specimen being merely an impression of the fossil as the mineralization (likely Calcite) has been dissolved away by ground water.

On another chunk of rock I found this which looks like it might also be a Crinoid arm. It needs a little prepping to tell for sure but sadly I didn't find any evidence in the field of a Calyx or body of the crinoid that belonged to this arm.

Yet another potential arm was found on yet another rock chunk.

There is good news and bad news that can be gleaned from these bits and pieces of Crinoids. On the one hand there is good evidence for a potential complete Crinoid. On the other is that the Mahantango formation tends to splinter easily and not form clean flat pieces. The three pieces shown above came from three different rocks in three different places. Maybe they are from the same layer or lens and it could be found again.

One last piece to show from this site was this neat cast of a large Crinoid segment. Check out the cool patterns, originally on the exterior of the stem, that have been preserved by the cast.

Tuesday, December 14, 2010

In places the Mahantango is fairly limited in fossil content and what is found is restricted to thin layers or lenses that feature a large quantity of shells. These are often referred to as "Hash" layers as the fossils are generally overlapping one another and picking one shell out of the crowd can be difficult. Usually these type of occurances form as lag deposits due to currents or maybe a storm washes a bunch of shells into a lower lying area. Whatever the case they can make for interesting looking assemblages. Here are a few that I've recently found at different locations within the Mahantango.

I found this piece on a pile but could not locate more. Front side has lots of Chonetid type brachiopods.

Back side has a Brachiopod/Pelecypod mix

This piece fascinated me when I saw this side. It's a current deposited pile of Chonetid type shells. Cross section view:

Head on view:

Another angle:

A mix of Spirifer and Chionetid shells with some slight distortion due to geologic pressures.

A mix of small Pelecypods. Some of the shells have a silvery coating on them... Hematite perhaps?

At one locality there is a layer of this limestone Coquina of broken shells. Likely formed along a surf zone along the coast. Evidence of temporary shallowing?

About Me

I'm a 40-something, life long student of Geology living in Philadelphia. My interests include Photography, Architecture and History among other things. When not cleaning my recent finds you'll find me in my backyard Hybridizing Daylilies or working in my garden. This Blog is an outlet for me to express my interest in fossils, catalog my collection and coalesce my thoughts and research into a useful medium.